Patent Documents 1 to 5 disclose seat structures formed by disposing such a cushioning member as a solid knitted fabric (a three-dimensional net member) on a seat cushion frame. When the cushioning member is disposed on the seat cushion frame in this manner, a base net such as a solid knitted fabric or a two-dimensional woven fabric is provided below the cushioning member via an elastic member in order to prevent bottom contact or improve vibration absorbing characteristic or impact absorbing characteristic. All the Patent Documents 1 to 5 disclose that a torsion bar unit provided with a torsion bar, an arm coupled to the torsion bar and supported to be rotatable about the torsion bar, and a supporting frame supported by the arm is disposed at a rear portion of a seat cushion and the base net is elastically supported by coupling a rear end of the base net to the supporting frame.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-347577    Patent Document 2: Japanese Patent Application Laid-open No. 2003-182427    Patent Document 3: Japanese Patent Application Laid-open No. 2004-188164    Patent Document 4: Japanese Patent Application Laid-open No. 2004-141545    Patent Document 5: WO2004/007238A1